1. Field of the Invention
The present invention relates to a zoom lens and an image pickup apparatus including the zoom lens, for use in a camera such as for example a broadcast television camera, a video camera, a digital still camera, or a silver film camera.
2. Description of the Related Art
In recent years, zoom lenses having a high aperture ratio, a high zoom ratio, a wide angle of view, and high optical performance have been demanded for image pickup apparatuses such as television cameras, silver film cameras, digital cameras, and video cameras.
In an image pickup apparatus such as a broadcast color television camera, since a color separation optical system and various filters are provided on the front side (object side) of an image pickup unit, a zoom lens is required to have a long back focus.
As a zoom lens having a high aperture ratio, a wide angle of view, a high zoom ratio, and a long back focus, a positive lead type four-unit zoom lens is known in which a lens unit having a positive refractive power is provided closest to the object side of the lens.
This four-unit zoom lens includes a first lens unit including a focus lens unit and having a positive refractive power, a second lens unit for zooming that has a negative refractive power, a third lens unit having a positive refractive power so as to correct image plane variation due to zooming, and a fourth lens unit for imaging that has a positive refractive power. The first to fourth lens units are arranged in that order from the object side to the image side.
Japanese Patent Laid Open Nos. 63-237020 and 2000-89109 and U.S. Pat. No. 6,545,818 disclose a positive lead type four-unit zoom lens having a high aperture ratio and a high zoom ratio. The four-unit zoom lens has an angle of view 2ω of 78° to 96° at a wide angle end, an f-number of 1.7 to 2.0, and a zoom ratio of 8 to 12.
In the above-described four-unit zoom lenses, it is necessary to properly set the refractive powers of the lens units and the lens layout in order to ensure a high aperture ratio, a high zoom ratio, and an angle of view 2ω of 78° or more at the wide angle end, and to achieve high optical performance over the entire zoom range.
For example, if the lens layout of the fourth lens unit for imaging is not optimized, it is difficult to reduce variations in chromatic aberration, spherical aberration, halo, coma and color difference of spherical aberration and to improve the zoom ratio and performance while ensuring a predetermined back focus.
In particular, when a lens unit (extender lens) for changing the focal length range of the entire system can be inserted and removed between an object-side lens unit and an image-side lens unit on the optical path of the fourth lens unit, it is important to properly set the lens layout of the image-side lens in the fourth lens unit.
If the lens layout of the image-side lens unit is not optimized, it is difficult to obtain a zoom lens that properly corrects lateral chromatic aberration at the wide angle end and that has high optical performance, a wide angle of view, and a high zoom ratio.